1. Field of the Invention
The present invention relates to an amplifier unit and a method of detecting failure in the same, specifically to a Doherty type amplifier unit having a carrier amplifier and a peak amplifier, and a method of detecting failure in such an amplifier unit.
2. Description of the Related Art
A Doherty type amplifier unit has drawn much attention as an amplifier suitable for power amplification in a base station of a mobile communications system due to its capability of operating efficiently. Such an amplifier is composed of a carrier amplifier and a peak amplifier. The carrier amplifier is typically biased for class A or AB operation. Thus, the carrier amplifier receives an input signal and amplifies the received signal to output the amplified signal even when the input signal is low.
The peak amplifier is biased for Class B or C operation. Thus, the peak amplifier does not amplify nor output any signal when electric power of an instantaneous input signal is not high enough to turn on the peak amplifier. In addition, the peak amplifier consumes no or negligibly low direct current power, which contributes to highly efficient operation of the Doherty amplifier as a whole.
When an instantaneous input signal to the peak amplifier is higher, the peak amplifier turns on and amplifies the input signal to output the amplified signal. The output signal is then synthesized onto the output signal from the carrier amplifier. Therefore, the Doherty amplifier can produce a higher saturation power.
In order to detect failure in the Doherty type amplifier unit in real time, it is being carried out to monitor changes in the total gain of the amplifier unit.
FIGS. 1A and 1B illustrate electrical characteristics of a Doherty amplifier composed of a carrier and a peak amplifier. Specifically, FIG. 1A illustrates an input-output characteristic and a gain characteristic when the Doherty amplifier is normally operating; and FIG. 1B illustrates the input-output characteristic and the gain characteristic when failure occurs in the carrier amplifier. It can be easily understood by comparing FIGS. 1A and 1B that gain is reduced significantly (by more than about 20 dB) when failure occurs in the carrier amplifier. Therefore, it is possible to detect failure in the carrier amplifier by monitoring changes in gain.
However, failure may occur in such an amplifier unit that cannot be detected by monitoring gain changes, as follows.
FIGS. 2A through 2C illustrate the input-output characteristics and the gain characteristics of the Doherty amplifier when failure occurs in the peak amplifier. Specifically, FIG. 2A illustrates the input-output and the gain characteristic when a partial failure occurs in the peak amplifier where a gate resistance of a Metal Oxide Semiconductor (MOS) transistor therein is reduced to about 39.4 Ohms; FIG. 2B illustrates the input-output and the gain characteristic when another partial failure occurs in the peak amplifier where the gate resistance of an MOS transistor therein is reduced to about 13.9 Ohms; and FIG. 2C illustrates the input-output and the gain characteristic when insulation breakdown occurs in the peak amplifier where the gate resistance is reduced to as low as about 0.8 Ohms.
As seen from FIGS. 2A through 2C, no significant reductions in gain are observed when failure occurs in the peak amplifier that operates below rated output, which makes it almost impossible to detect failure in the peak amplifier only by monitoring gain changes. Therefore, there is required other means for detecting failure in the Doherty amplifier, especially, in the peak amplifier thereof.
Although lots of prior art documents including patent-related documents listed below about the Doherty amplifier are available, there are only some that disclose measures against failure for the purpose of higher reliability in commercial use. As one of the examples, patent-related document 2 discloses a Doherty amplifier having a control circuit. When one of the carrier and the peak amplifier in the Doherty amplifier fails, the control circuit detects presence or absence of output signals from the carrier and the peak amplifier and accordingly controls a bias voltage of either one of the amplifiers that is outputting a signal, in order to reduce non-linear distortion of the output signal.
The patent-related document 2 discloses, in other words, a failed-amplifier identification circuit that detects presence or absence of output signals from the carrier and the peak amplifier so as to determine the amplifier outputting no signals as a failed amplifier. In such a circuit, signal levels outputted from the amplifiers are compared with a predetermined threshold level in order to determine which amplifier fails. Therefore, the control circuit has to be composed of increased numbers of circuit elements or components, which may cause false detections due to variations in electrical properties of those elements or the like. Therefore, a circuit configured simply with fewer circuit elements or components has long been desired in order to detect failure in a transistor device of the amplifier.
[Patent-related document 1] Japanese Patent Application Laid-Open Publication No. H9-181533
[Patent-related document 2] Japanese Patent Application Laid-Open Publication No. 2006-60301
When the carrier amplifier fails in the Doherty type amplifier unit, a significant reduction in gain is observed, which makes it possible to detect the failure by detecting reductions in gain or output power. However, it is difficult to detect failure by detecting reductions in gain or output power when the peak amplifier fails since such failure causes only slight reduction in gain. This is because the peak amplifier biased for Class B or C operation produces lower gain in normal operation, whereas the carrier amplifier biased for Class A or AB operation produces higher gain.
The present invention provides an amplifier unit and a method for detecting failure, which can certainly detect failure that may occur in the peak amplifier, using a simplified configuration. Furthermore, the present invention also provides an amplifier unit and a method for detecting failure, which can detect failure that may occur in other portions in the amplifier unit.